Abstract The effect of Me4NI, n-Bu4NI, n-BuEt3NI, Et3PhNI and Et4NBez on the temperature of maximum density of water was determined using a dilatometric technique. The concentration range was from 0.005m to 0.050m in all cases. The temperature data was used to calculate Despretz constants using the equations ΔΦ=Φ sot - Φ w and ΔΦ=K m m where Φ sot - Φ w is the difference between temperature of maximum density of the solution and of water, Km is the Despretz constant. The equation of Kaulgud and Pokale ΔΦ+1,074 m 3/2=Km was also used. All the calculated K m 's are negative, that is to say, the effect of the salts is to lower the temperature of maximum density. The individual ion constants were calculated using literature values for the iodide and the tetraethyl ammonium ions. The order followed by the absolute values of the constants is Me4N+ < n-BuEt3N+ < Et3PhN+ < BuN+. The results confirm the proposal of Kaulgud and Pokale in the sense of having the quaternary ammonium ions conserving their influence on the structure of water for temperatures in the neighborhood of that of maximum density of water.